Vapor generator



Nov. 3, 1964 E. M. POWELL ETAL 3,155,075

\ VAPOR GENERATOR Filed Dec. 27. 1961 2 Sheets-Sheet 1 INVENTORS:

ELNO M. POWELL BY WILLBURT W. SCHROEDTER [ML/5Z- 95% ATTORNEY 1964 E. M. POWELL ETAL 3,155,075

VAPOR GENERATOR 2 Sheets-Sheet 2 Filed Dec. 27, 1961 WITHOUT GAS RECIRCULATION PER CENT OF LOAD FlG. 2

INVENTORs:

ELNO M. POWELL WILLBURT W. SCHROEDTER ATTORNEY United States Patent 3,155,675 VAPOR GENERATOR Elno M. Powell, Bloomfield, and Willburt W. Schroedter,

West Hartford, Comp, assignors to Combustion Engineering, Inc, Windsor, Comm, a corporation of Delaware Filed Dec. 27, 1961, Ser. No. 162,329 6 Claims. (Cl. 122-479) This invention relates generally to high temperature, high pressure vapor generators and has particular relation to an improved generator organization and method of operating the same wherein the double reheat cycle is employed together with gas recirculation type of control.

In accordance with the invention the various heat exchange surfaces or sections of the vapor generator and particularly the surfaces of the two reheats, i.e., the high pressure reheat and the low pressure reheat, are organized in such a manner that the optimum control effect is obtained by means of the gas recirculation control insofar as regulating the two reheat temperatures with varying load is concerned. This result is achieved by arranging the heat excahnge surfaces of the two reheats with relation to each other and with relation to the other heat exchange surface of the vapor generator in such a manner as to take maximum advantage of the characteristic efiect of gas recirculation control and the operating characteristics of the two reheats of double reheat unit. It is the characteristic of gas recirculation to be progressively more effective in influencing a change in heat absorption in zones located progressively further from the furnace outlet or progressively further downstream in a gas flow direction. It is further the characteristic of many double reheat units that the high pressure reheater requires a progressively higher percentage of the total heat absorption of the unit as the load is progressively decreased and as contrasted with the low pressure reheater which will require a much lower percentage increase or may require little or no increase in its percentage of heat absorption with relation to the total heat absorption as the load is changed.

In accordance with the invention there is provided a vapor generator of these operating characteristics that includes a fluid cooled furnace, preferably having primary fluid carrying heat exchange surface lining its walls, with the furance being fired with a suitable fuel and having an off-take in the form of a gas pass which extends therefrom and with this gas containing various heat exchange surfaces of the vapor generator. Near the outlet of this gas pass there is provided a gas recirculation conduit which has connected therein a recirculating fan the outlet of which communicates with the furnace so as to recirculate combustion gases to the furnace for vapor temperature control purposes.

The generator is operated on the double reheat cycle, having both a high pressure and a low pressure reheater with these reheaters being disposed so that the combustion gases generated in the furance pass serially thereover and impart heat to the reheat vapor. The high pressure reheater has a substantial portion (i.e. major portion) of its surface located well downstream in a gas flow direction with relation to the low pressure reheater and with there being interposed in the gas stream between the location of the low pressure reheater and this downstream portion of the high pressure reheater additional heat exchange surf-ace which is preferably vapor heating surface for the primary circuit of the vapor generator.

The generator may be designed so that at maximum load gas recirculation is not needed for control purposes with the temperature of the high pressure and the low pressure reheat vapor leaving the respective reheaters being at its desired value. At maximum load there may be a small amount of gases recirculated so that the recirculation fan is tuning and hot and back flow therethrough is prevented. As the load is decreased from the maximum load however, if there were no compensating control action utilized the high pressure reheat temperature would fall from its desired value at very much faster rate than the low pressure reheat temperature which would also decrease from its desired value. To compensate for this tendency, combustion gases are returned to the furnace by means of the gas recirculation system with the quantity of gases so returned being progressively increased as the load is decreased. This recirculation of combustion gases increases the heat transfer to the high pressure and low pressure reheaters, as well as other heat exchange surface disposed in the combustion gas stream, above that which would prevail Without such gas recirculation with a given firing rate of the generator. This increase in heat absorption becomes progressively greater as the amount of gas recirculation is increased up to a given limit.

Furthermore this increase in heat absorption is substantially greater at downstream locations in the gas stream than it is at upstream locations with the increase that is produced becoming progressively greater toward the downstream and of the gas stream. By positioning the low pressure reheater at a location in the gas stream well upstream of a substantial portion of the high pressure reheater and by positioning primary or other heat exchange surface intermediate the low pressure reheater and this downstream portion of the high pressure reheater, this elfect of the gas recirculation type of control to have more pronounced or gerater control action on the downstream heat exchange sections may be utilized to substantially completely or to a large extent overcome the difference in heat absorption characteristics of the two reheats, i.e., the characteristic of the high pressure reheat to require an increasing percentage of the total heat absorption relative to that of the low pressure reheat with decreasing load in order that the two reheat temperatures will be maintained at a generally constant and desired value.

Accordingly it is an object of the invention is to provide an improved vapor generator operating on the double reheat cycle.

It is another object of this invention to provide such an improved vapor generator employing gas recirculation with the arrangement being such as to obtain optimum control of the reheat temperatures by means of the gas recirculation control.

A still further object of the invention is to provide an improved method of operating a high temperature, high pressure vapor generator which is operating on the double reheat cycle and with gas recirculation control for the reheat temperatures being employed.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to obtain the results desired as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

FIG. 1 is a diagrammatic representation in the nature of a vertical sectional view through a high capacity, high pressure vapor generator employing the present invention; and

FIG. 2 represents a set of curves showing the variation in percent of the total heat absorption with varying load by (a) the furnace, (b) the high pressure reheat and (c) the low pressure reheat both without gas recirculation and with gas recirculation control.

Referring now to the drawing, wherein like reference characters are used throughout to designate like elements, the illustrative and preferred embodiments of the invention as depicted therein includes the furnace lid which, as disposed, is vertically elongated having its outlet for combustion gases identified as 12 at its upper end from which outlet extends the gas pass 14 that conveys the combustion gases egressing from the furnace first horizontally and then downwardly to the outlet 16 with the gases from this outlet preferably being conveyed to a suitable air heater and then to a stack. The furnace is fired through the burners 18 which, in the arrangement disclosed, are, so-called, tangentially arranged burners operative to create a whirling mass of burning fuel in the furnace rotating about the vertical axis of the furnace. These burners are supplied with fuel and air from a source not disclosed in the drawing.

The furnace It has its walls lined with vertically extending tubes which form a part of the primary circuit of the vapor generator. These tubes are connected at their lower ends to suitable headers 20 and at Lheir upper ends to headers 22.

The primary fluid of the vapor generator is forced up through the vertically disposed tubes 24 with the fluid being conveyed down through conduit 28 and through pump 26 to the entrance of these tubes. After the primary fluid has traversed the tubes 24 it is conveyed to the chamber 30 from the headers 22 via the connecting conduits 32. Preheated feedwater is also conveyed to the chamber 30 through and by we}? of the economizer 34 which is positioned at the lower end of the gas pass 14 and which is comprised of a heat exchange bundle or bundles in the form of sinuously bent tubes, with a feed pump (not disclosed) being disposed upstream of the economizer.

Primary fluid, having heat imparted thereto via passage through tubes 24, is conveyed from the chamber 3%) to and through various vapor heat exchange sections or surfaces which are placed in the path of the combustion gases generated in and passing from the furnace It In the illustrative arrangement the primary vapor is conveyed through conduit 49 to the low temperature primary vapor heating section 42 which is comprised of tubular panels in spaced relation across the gas stream with each of the panels being comprised of sinuously bent tubes. From this low temperature section the primary vapor is conveyed via conduit 44 to the intermediate temperature heat exchange section 46. From this intermediate heat exchange section the primary vapor is conveyed via conduit 48 to the heat exchanger 50, which will be described later, and from this heat exchanger 50 the primary vapor is conveyed to the high temperature heat exchange section 52 where its temperature is raised to the desired value with the primary vapor being supplied to a turbine through the conduit 54.

After the primary vapor has passed through the high pressure portion 5a of the turbine machine it is returned to the vapor generator for reheating in the high pressure reheater to a desired predetermined temperature. This high pressure reheater, in' the illustrative embodiment, is comprised of two sections, a low temperature section 58 and a high temperature section 69. The vapor to be reheated is conveyed from the turbine portions 56 through conduit 62 with this vapor first passing through the heat exchanger 50 and then entering the low temperature section 58 of the high pressure reheater. From this section 58, which is located downstream with regard to the gas flow of the intermediate temperature heat exchanger 46 for the primary vapor, this reheat vapor is conveyed through conduit 64 to the high temperature section of the high pressure reheater. After traversing this high temperature section 60 to reheat vapor is heated to its desired temperature and it is then conveyed to the intermediate pressure portion 66 of the turbine.

After traversing this intermediate pressure turbine pertion 66 the vapor is reconveyed to the vapor generator for again being reheated to a desired temperature with the vapor being conveyed through conduit 68 to the low pressure reheater 79. After traversing the low pressure reheater the vapor is reheated to its desired temperature and is then conveyed through conduit 72 to the low pressure portion '74- of the turbine machine. As disclosed, the low pressure reheater 70 is positioned adjacent to the high temperature portion 6% of the high pressure reheater and is positioned well upstream of the low temperature portion 58 of this high pressure reheater with the heat exchangers 42 and 46 of the primary vapor heating surface being interposed between the portion 58 of the high pressure reheater and the low pressure reheater '70.

By arranging the heat exchange surface in the manner disclosed the optimum results may be achieved in connection with controlling the two reheat temperatures by means of gas recirculation. The generator of the invention is provided with a gas recirculation system which includes the duct '76 which leads from the lower end of the gas pass 14 to the ash discharge chamber 78 at the bottom of the furnace. Connected within this duct '76 is the recirculation fan 84) which is effective to withdraw gas from the gas pass 14 and introduce it into the chamber 7'8. The amount of gases thus recirculated may be controlled either by controlling the operation of the fan 80 through a variable speed motor or by means of a damper arrangement illustrated generally as 82.

As previously mentioned it is a characteristic of the vapor generator of the invention that of the total percentage of heat absorption of the unit the percentage required by the high pressure reheat increases substantially as the load decreases in order to maintain the outlet temperature of the high pressure reheat vapor at its desired and generally constant value while the percentage required of the low pressure reheater increases to a much less extent than the high pressure reheater or does not increase or may even decrease somewhat depending on the arrangement of the system including the turbine hookup. While the actual amount of the percentage change of the high pressure reheat heat absorption and while any change in the low pressure heat absorption characteristic will vary somewhat depending upon the particular turbine installation and cycle employed the aforementioned characteristic is provided with most units, and it is with units having this characteristic with which the invention is concerned. In addition to this characteristic with relation to the reheat heat absorption it is a characteristic of vapor generators which have their furnaces cooled by heat exchange tubes that as the load is reduced an increase in the percentage of the total heat absorption of the unit takes place in the furnace.

FIG. 2, in the solid line curves, illustrates the percent of heat absorption with varying load that would prevail if there were no ga recirculation control employed. The solid line curve identified as furnace shows that the percentage of heat absorption with relation to the total heat absorption would steadily increase at a rather sharp rate as the load is decreased. The solid line curve identified as high pressure reheat shows that without gas recirculation control the percentage of the total heat absorption of the unit which is imparted to the high pressure reheat would fall as the load decreases. This of course is just contrary to the characteristic of the high pressure reheat which requires that the percentage of heat absorption increase with decreasing load in order to maintain a constant reheat temperature for the high pressure reheat. The solid curve identified as low pressure reheat shows that the percentage of the total heat absorption which is imparted to the low pressure reheat without gas recirculation control would fall as the load is decreased with this causing the temperature of the low pressure reheat to decrease with a decreasing load since the low pressure reheat requiresa generally constant percentage of the total heat absorption of the unit.

By means of the gas recirculation control and the distribution of the various heat exchange surfaces in the manner described the temperature of the high pressure and the low pressure reheat may be maintained grenerally constant over a predetermined load range by means of the gas recirculation control.

The dotted line curves of FIG. 2 indicate the percentage of heat absorption which is obtained in the furnace and in the two reheat-s by means of the gas recirculation control or more correctly in the unit when it is provided with a gas recirculation control. The dotted line curve identified as furnace shows that the heat absorption in the furnace is decreased somewhat as the load is decreased. The dotted line curve identified as high pressure reheat" shows that the percentage of heat absorption with rela tion to the total heat absorption of the unit is increased for the high pressure reheater with a decrease in load which is necessary to maintain the high pressure reheat at its desired and generally constant value with decrease in load. The dotted line curve identified as low pressure reheat indicates that with gas recirculation the percentage of the total heat absorption that is supplied to the reheat remains generally constant with a decrease in load (with a slight decrease being had in the illustrative example chosen and with this being nemssary to maintain the low pressure reheat outlet tempera-ture at its desired and generally constant value.

The increase in percentage of total heat absorption that is achieved for the high pressure reheat while the percentage of the total heat absorption for the low pressure reheat remains generally constant is possible by means of positioning the low pressure and the high pressure reheat surface in the gas pass in a manner to achieve this result utilizing the characteristic of the gas recirculation control to be progressively more eifective at locations progressively further downstream in the gas flow sense. The further downstream in the combustion gas stream a heat exchange surface is placed the greater change in heat absorption that will be eflected by means of gas recirculation. Thus in the illustrative vapor generator the greatest change in heat absorption due to the gas recirculation will be produced in the economizer. The next greatest change will be produced in the low temperature section of the high pressure reheater with the change in heat absorption becoming progressively less in the heat exchangers 46, 42, '76, and 65 working backwards in regard to the direction of gas flow. Since the low temperature section of the high pressure reheater is spaced sub stantially downstream of the low pressure reheater 70 the increase in heat absorption as a result of gas recirculation control will be much greater in the high pressure rehe-ater.

Since the low temperature section 58 of the high pressure reheater is a very substantial portion of the total heat exchange surface of the high pressure reheater and since the low temperature section 58 is located well downstream in a gas flow sense from the low pressure reheater 78 with heat exchange portions 42 and 46 of the primary vapor heating surface being interposed in the gas stream between these two reheat heat exchangers and further since thehigh temperature section of the high pressure reheater is located close or immediately adjacent the low pressure reheater the gas recirculation control is much more effective with relation to the high pressure reheater than the low pressure reheater.

It is essential tothe invention that a very substantial portion of the high pressure reheater be spaced well downstream in a gas flow sense from the low pressure reheater and that there be heat exchange surface positioned between this substantial portion of the high pressure reheater and the low pressure reheater. ,It is further essential that the high temperature portion of the high pressure reheater be located close to the low pressure reheater. This is so since if the low pressure reheater were spaced substantially downstream from the high temperature portion ofthehigh pressure reheater the efi'ect of spacing the low temperature section of the high pressure reheater well downstream of the low pressure reheater would be effectively nullified.

By means of carefully locating the high pressure reheater surface with relation to the low pressure reheater surface as described and carefully sizing the various portions of these heat exchangers it is possible to provide a unit wherein, by means of gas recirculation control, both the high pressure and the low pressure reheat temperatures may be maintained generally constant as the load decreases through a substantial range from its maximum value.

In order to make less intricate the problem of design and in order that the reheat temperatures may be maintained even though slag deposits cause a substantial variation in heat absorption from that of the design value the unit may be provided with a heat exchanger 50 which is effective to exchangeheat between the primary fluid and the high pressure reheat vapor with the unit when utilizing such a heat exchanger being designed such that the temperature of the low pressure reheat is maintained at its desired value by gas recirculation while the final desired temperature of the high pressure reheat is cont-rolled by means of the heat exchanger 50. It is noted that with the invention the most that is required of this heat exchanger 50 is a relatively minor control with the major control effect being provided in both of the reheats, i.e., the high pressure reheat and the low pressure reheat by means of gas recirculation. If it desired to provide a control by means of a heat exchanger such as that of 543 wherein heat is transferred from the primary fluid to the high pressure reheat fluid the high pressure and low pressure reheat surfaces are so sized that with the low pressure reheat being maintained at its desired value by means of gas recirculation the high pressure reheat will be slightly under its desired value without any heat exchange from the heat exchanger 56 so that this heat exchanger may be operated and controlled to give this desired slight increase in high pressure reheat temperature.

The heat exchanger 50 may be constructed so that tubes identified as 82 are positioned within tubes identified as 84 with the high pressure reheat vapor being conveyed in the annulus between these two tubes while the primary fluid is conveyed through the tubes 82. The bypass 86 together with the valves 88 provide a means of controlling the amount of heat imparted from the primary fluid to the reheat fluid. In an organization utilizing this heat exchanger 50, the design may be such that at maximum load, all of the primary fluid passes through the bypass 86 and none through the tubes 82 while as the load d creases and gas recirculation is progressively increased to control the two reheat temperatures a small amount of the primary fluid may be conveyed through the tubes 82 to bring the high pressure reheat vapor up to its final desired value.

To eflect this desired control there may be provided temperature responsive device 90 which responds to the outlet temperature of the low pressure reheater and which actuates a'suitable control through motor 92 to control the quantity of recirculating gases admitted to the furnace in order to maintainthis temperature a its desired and generally constant value. In addition there may be a temperature responsive device 94 responsive to the outlet temperature of the high pressure reheat and effective to control the control device )6 to regulate the valves 88 in a manner to maintain the temperature of this low pressure reheat vapor at its desired value.

Accordingly it will be seen that by means of the present invention an improved vapor generator organization operating on the reheat cycle is provided together with an improved method of operating the same wherein the optimum control effect which may be provided by means of gas recirculation is had in connection with controlling the two reheat temperatures.

It is noted that the present invention is directed to an apparatus for controlling the two reheats of a double reheat power plant system with the invention not being concerned with the control of the temperature of the primary fluid delivered to the high pressure stage of the turbine machine. The control of this high pressure fluid may be provided in a number of ways such by means of desuperheating or by means of a separately fired high pressure vapor heater, or in connection with a supercritical unit the temperature of the high pressure vapor is, of course, controlled by means of adjusting the firing of the unit primarily and also adjusting other parimeters, with the invention being applicable to both natural and forced circulation units including supercritical oncethrough units.

While we have illustrated and described a preferred embodiment of our invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within the purview of our invention.

What we claim is:

l. A vapor generating unit operating on the double reheat cycle, said cycle having the characteristic that of the total heat absorption of the generator the percentage required by the high pressure reheat increases relative to that of the low pressure reheat as the load decreases comprising a furnace fired with a suitable fuel and having primary fluid carrying tubes on its Walls, a passageway extending therefrom for the conveyance of combustion gases, a high pressure and a low pressure reheater disposed in said gas passageway, the high pressure reheater having its major portion spaced well downstream of at least the major portion of the low pressure reheater and disposed so at least a vast majority of the gas stream passes thereover, other heat exchange surface disposed in the passageway between these portions, and means for controllably reintroducing into the furnace combustion gases that have traversed said reheaters and in a manner operative to increase the heat content of the gases traversing said reheaters, the downstream portion of the high pressure reheater being such that the gas recirculation control is more effective with relation thereto than with the low pressure reheater whereby the tendency of the reheat temperatures to vary relative to each other is compensated for at least in part.

2. A vapor generator operating on the multiple reheat cycle and having the characteristic that of the total heat absorption of the generator the percentage required by a high pressure reheater increases relative to that of a low pressure reheater, said generator having a fluid cooled furnace with an oiftake therefrom, means for firing said furnace generating a stream of combustion gases that pass therethrough and through said oiftake, primarily convection high pressure reheat means, primarily convection low pressure reheat means, said two reheat means being serially arranged with respect to combustion gas flow and disposed in the gas stream so that with regard to the total heat absorption of each the high pressure reheater means effectively absorbs its heat from a location downstream of the low pressure reheater means with the high pressure reheater means having the major portion of its heat exchange surface downstream of at least a vast majority of the heat exchange surface of the low pressure reheater means and with other heat exchange surface disposed therebetween in the gas stream, said major portion of the high pressure reheat means being disposed in the gas stream so that at least a major portion of the stream passes thereover, and means operative to recirculate combustion gases to said furnace in a i g manner to increase the heating effect of the gas stream with this effect being more pronounced at locations progressively further downstream and with the location of the effective heat absorption of the high pressure reheater being sufliciently downstream of the low pressure reheater so as to fulfill to a large extent the requirement of the high pressure reheater to have an increased percentage of the total heat absorption relative to the low pressure reheater with decreasing load in order to maintain the reheat temperatures at their desired value.

3. A vapor generator operating on the double reheat cycle with a characteristic that of the total heat absorption of the generator the percentage required by the high pressure reheat increases relative to that of the low pressure reheat as the load decreases, said generator comprising a furnace having an oiftake adjacent one end, means for firing the furnace remote from said outlet, said furnace having its walls lined with fluid cooling tubes, a high pressure reheater disposed in the path of the combustion gases generated by said burning fuel, a low pressure reheater disposed in said combustion gas pass, means for controllably introducing combustion gases which have traversed said reheaters into said furnace at a location such that the heat transfer to heat exchange surface over which this gas stream passes is increased with the increase in heat transfer caused by this gas recirculation increasing progressively along the gas stream, said high pressure reheater having a high temperature portion at an upstream location of at least a major portion of the low pressure reheater and a low temperature portion at a location downstream of at least a portion of the low pressure reheater that comprises a great preponderance of the whole low pressure reheater, said low temperature portion of the high pressure reheater having a major portion of the combustion gas stream passing thereover, primary heat exchange surface interposed in the gas stream intermediate this low temperature portion and the predominant portion of the low pressure reheater, the location of the downstream or low temperature, high pressure reheater portion being in such relation to the low pressure reheater that the tendency of the high pressure reheater temperature to decrease relative to the low pressure reheat temperature with decrease in load is substantially eliminated or greatly reduced.

4. A vapor generator comprising an upright furnace having its walls lined with tubes carrying the primary fluid, means introducing fuel into the furnace for burning therewithin, means conveying combustion gases from the upper region of the furnace, a high pressure reheater disposed in said combustion gas stream, a low pressure reheater disposed in said combustion gas stream, means for controllably introducing combustion gases which have traversed said reheaters into the lower region of the furnace, said high pressure reheater having a low temperature stage and a high temperature stage in series flow relation, with the low temperature stage comprising a major portion of the high pressure reheater and disposed so generally the entire gas stream passes thereover, this low temperature stage being at a location in the gas stream well downstream of at least a portion of the low pressure reheater that comprises a great preponderance of the Whole low pressure reheater, fluid heating surface carrying primary fluid in the gas stream intermediate the low temperature stage of the high pressure reheater and said portion of the low pressure reheater and forming part of the primary fluid carrying circuit of the generator.

5. A power plant system wherein a vapor generator supplies the motive fluid for a turbine with the system operating on the double reheat cycle having the character'i stic that of the total heat absorption of the generator the percentage required by the high pressure reheat increases relative to that of the low pressure reheat as the load decreases in order to maintain the reheat temperatures at their desired value, said generator including a fluid cooled furnace fired with a suitable fuel and having an outlet for combustion gases, a gas pass extending from said outlet, low pressure and high pressure reheat means disposed in said gas pass such that at least a major portion of the combustion gases produced in the furnace pass thereover with the efiective location of high pressure reheat means being such that of the total heat absorption of the high pressure reheat means it receives a predominant portion downstream relative to combustion gas flow of the effective location at which the low pressure reheat means receives the predominant portion of its heat absorption, primary heat exchange surface disposed in the gas pass between a low pressure reheat portion and a high pressure reheat portion downstream of this low pressure reheat portion, means for controllably recirculating gases to the furnace at a location remote from said outlet and operative to increase the same with decrease in load to thereby increase the heat input to both reheat means with that of the high pressure reheat means being increased to a greater extent than that of the low pressure reheat means and means independent of the firing of the generator effective to adjust one of the reheat temperatures relative to the other.

6. A double reheat vapor power plant cycle having the characteristic that as the load decreases from maximum and in order to maintain the two reheats at their desired value the percentage of the total heat absorption required by the high pressure reheat increases relative to that required by the low pressure reheat characterized by a vapor generator including a furnace having a combustion gas exit and primary fluid carrying tubes on its walls, means firing said furnace, a gas pass extending from said exit, high pressure and low pressure reheats in said gas pass, means operative to controllably introduce combustion gases which have traversed said reheats into the furnace at a location remote from said outlet so that such gas introduction has its greatest effect in changing heat absorption of heat exchange surface in the gas pass at locations furthest downstream relative to gas flow, said high pressure reheat having at least a substantial portion of its surface downstream relative to gas flow of at least a portion of the low pressure reheat, this latter portion comprising a vast majority'of the low pressure reheat, primary heat exchange surface interposed between these last-mentioned high pressure and low pressure reheat portions, said substantial portion of the high pressure reheat being disposed so that a vast majority of the combustion gas stream traverses the same and having a heat absorption capability such that an increase in gas recirculation with decrease in load increases the heat input of both reheats with respect to the percentage of their heat absorption relative to the total heat absorption of the generator and increases that of the high pressure reheat to a greater extent than that of the low pressure reheat, and means responsive to the final low pressure reheat temperature operative to regulate the introduction of combustion gases.

References Cited in the file of this patent UNITED STATES PATENTS 2,897,797 Koch Aug. 4, 1959 2,918,909 Nickel Dec. 29, 1959 2,966,896 Vogler Jan. 3, 1961 3,002,347 Sprague Oct. 3, 1961 3,035,556 Brunner May 22, 1962 FOREIGN PATENTS 793,048 Great Britain Apr. 9, 1958 W m V,

UNITED STATES PATENT OFFICE Certificate Patent No. 3,155,075 Patented November 8, 1964 Elno M. Powell and Willburt W. Schroedter Application having been made jointly by Elno M. Powell and Willburt W. Schroedter, the inventors named in the patent identified above, and Combustion Engineering, Inc., of Windsor, Conn, a corporation of Delaware, the assignee, for the issuance of a certificate under the provisions of Title 35, Section 256 of the United States Code, deleting the name of the said Elno M. Powell from the patent as a joint inventor, and a showing and proof of facts satisfying the requirements of the section having been submitted, it is this 20th day of January 1970, certified that the name of the said Elno M. Powell is hereby deleted from the patent as a joint inventor with the said WVillburt W. Schroedter.

LUTRELLE F. PARKER Law Ewe/miner. 

1. A VAPOR GENERATING UNIT OPERATING ON THE DOUBLE REHEAT CYCLE, SAID CYCLE HAVING THE CHARACTERISTIC THAT OF THE TOTAL HEAT ABSORPTION OF THE GENERATOR THE PERCENTAGE REQUIRED BY THE HIGH PRESSURE REHEAT INCREASES RELATIVE TO THAT OF THE LOW PRESSURE REHEAT AS THE LOAD DECREASES COMPRISING A FURNACE FIRED WITH A SUITABLE FUEL AND HAVING PRIMARY FLUID CARRYING TUBES ON ITS WALLS, A PASSAGEWAY EXTENDING THEREFROM FOR THE CONVEYANCE OF COMBUSTION GASES, A HIGH PRESSURE AND A LOW PRESSURE REHEATER DISPOSED IN SAID GAS PASSAGEWAY, THE HIGH PRESSURE REHEATER HAVING ITS MAJOR PORTION SPACED WELL DOWNSTREAM OF AT LEAST THE MAJOR PORTION OF THE LOW PRESSURE REHEATER AND DISPOSED SO AT LEAST A VAST MAJORITY OF THE GAS STREAM PASSES THEREOVER, OTHER HEAT EXCHANGE SURFACE DISPOSED IN THE PASSAGEWAY BETWEEN THESE PORTIONS, AND MEANS FOR CONTROLLABLY REINTRODUCING INTO THE FURNACE COMBUSTION GASES THAT HAVE TRAVERSED SAID REHEATERS AND IN A MANNER OPERATIVE TO INCREASE THE HEAT CONTENT OF THE GASES TRAVERS- 